A wide range of battery technologies exist today. However there is considerable demand for batteries that deliver much higher performance. In particular, higher energy densities, and/or higher available power for a given energy density, are desirable. Energy density may be expressed in terms of energy per unit volume, energy per unit mass and energy per unit area. Different product applications prioritise each of these differently.
Current battery technology may be described as planar or essentially 2-D. A schematic diagram of a current technology lithium ion battery is shown in FIG. 1. Lithium ions travel from one electrode to the other. The direction of travel changes depending on whether the battery is charging or discharging. The cathode electrode 10 is made up of a lithium insertion material, e.g. lithium manganese oxides, in the form of particles 12. These particles are mixed with carbon-based particles 14 which enhance conductivity, and the particle mix is bound using a polymer binder.
For a lithium ion to travel from anode 18 to cathode 10, it must first diffuse out of a carbon particle and through the gaps in the carbon particles of the anode, then across the ˜25 μm electrolyte gap/separator 20 gap, then through the mix of carbon particles and lithium manganese oxide particles, then actually insert into a lithium manganese oxide particle. An electrode may be up to 100 μm thick. Thus a lithium ion may have to travel up to 100 μm through the mix, then 1-3 μm into the oxide particle.
Lithium ion transport is critical to the operation of the cell. The more difficult the transport, the higher the internal resistance. This resistance can limit performance, by reducing charge/discharge rates and increasing heat.
Electron transport is also critical. In FIG. 1, electrons must move in and out of the particles of lithium manganese oxide, which is the active electrode material. Such materials can have poor electronic conductivity. The electrons must then move through a tortuous, discontinuous path of particles of lithium manganese oxide and carbon particles. This resistance can also limit performance, by reducing charge/discharge rates and increasing heat.
So-called 3-D electrodes and 3-D batteries have been proposed as a way of circumventing the problems associated with planar technology to give higher performance. A 3-D electrode is shown in FIG. 2. In this diagram, lithium ions can move freely through the electrolyte over the whole electrode thickness, i.e. there is no particle mix to wind their way through. The lithium insertion material, e.g. lithium manganese oxide, is present as a very thin coating (e.g. 100 nm) on conducting nano-cylinders 32. Thus the greatest distance the lithium ion has to insert into the lithium manganese oxide is only the thickness of the coating. Furthermore, the conducting pathway is continuous, and provides a much better route for electron migration. Electron movement through the lithium manganese oxide is also minimised.
This concept is further developed with the complete 3-D battery, shown in FIG. 3. The 3-D battery has interconnected 3-D electrodes, being anode 34 and cathode 36 that are separated by a very thin film of electrolyte 38. This minimises the distance the lithium ion must travel, further decreasing internal resistance.
3-D batteries have proven difficult to construct. A commonly used approach with the electrodes is to use cylinders that are grown from an electrode into a porous template (shown in FIG. 4A). to form a filled template (FIG. 4B) The template is removed to leave the cylinders (FIG. 4C). This process is awkward and difficult to scale, and there are problems with removing the template while keeping the cylinders intact. Also, to achieve sufficient surface area, the cylinders should be very thin. Since battery electrodes are normally relatively thick (˜100 μm) the cylinders may have a very high aspect ratio. Thin fibres of high aspect ratio have been known to cause asbestosis.
Another way of making a 3-D electrode is to first make nanowires, then chop them and combine somehow (e.g. conductive binder, heat) to form the electrode. This method still has the previous safety concerns over nanowires, and also it can be difficult to make nanowires at scale.
Other methods involve applying a coating of active material to conducting metal meshes. By active material, we mean a material that is able to transform between charged and discharged states. The higher the number of charge/discharge transformations that the material can withstand, the higher the stability of the material. U.S. patent application 2010/0035153A1 to Thackeray et. al. describes battery anode materials made by electrodepositing various tin alloys onto copper mesh. Such mesh materials are very coarse structures and have extremely low surface areas. The disadvantage of low surface area materials is that thicker coatings must be applied in order to achieve reasonable energy capacities. Thicker coatings have diminished rate characteristics compared to thinner coatings. By rate characteristics, we mean the ability to charge and discharge at high rates. Materials with good rate characteristics can charge and discharge at high rate without excessive resistance effects such as heat build up and losses. The metal meshes are examples of substrates that are themselves conducing. Thus the coating by itself, need not provide a continuously conductive path, however for thicker coatings reasonable conductivity may be desirable.
Other methods include impregnating or filling voids between polymer spheres or arrays of polymer spheres such as ‘inverse opal’ structures. However these are also expensive and difficult methods and generally do not provide high surface area substrates.
Further methods include coating a polymer foam material with a metal, then removing the polymer to create a metal foam, then coating with metal foam with active material. However such metal foams can be expensive, they are generally quite thick (>1 mm) and have low surface areas.
Carbon aerogels have also been utilised as a conducting substrate to support MnO2. However carbon aerogels are expensive, can contain a lot of very small pores that are inaccessible to ions, and generally contain quite high volume fractions of carbon, leaving less room for active material. Carbon aerogels are an example of a conductive substrate. Again, in this instance the coating, by itself, need not provide a continuously conductive path.
There is clearly a need for battery materials and methods that enable simpler, more cost effective and safe 3-D battery materials.
The present inventors have surprisingly found that it is possible to manufacture 3-D batteries using various porous polymeric materials as a framework for deposition of materials to form an interconnected array of materials that can provide battery electrodes. The invention relates to such electrodes and to batteries incorporating such electrodes.